Preliminary investigations in our laboratory have repeatedly demonstrated suboptimum growth rates for rats fed a diet containing vitamins and minerals at levels equal to the rat's nutrient requirements (as estimated by the National Research Council). When rats fed the same diet with twice the required levels of vitamins and minerals, growth rates were increased by 35% and were approximately equal to the growth rates of rats fed a natural-ingredient diet. These results have been recently confirmed in a manner of independent laboratories, each study indicating nutrient-limited performance resulting from supplies of one or more nutrients equivalent to previous estimates of nutrient requirements. We propose to determine which nutrients are deficient in the published nutrient requirements. We believe that it is important to test minimum levels of all nutrients in a new formula because removal of excess amounts of nutrients eliminates their potential sparing effect on the requirements of other interrelated nutrients. The apparent inadequacy of the NRC's requirements may be due to nutrient sparing effects obscuring the full nutrient need when excess sparing factors are not available. An experimental design of systematic addition and depletion is described. This technique is designed to identify the limiting nutrients in the NRC's list of nutrient requirements, even if nutrient interactions are occurring. The technique may also reveal new nutrient interactions. Additional preliminary data indicate that existence of a 3-way interaction between Na-K-Cl, and the nutritional requirements for each of these elements appears to be underestimated. We shall confirm and describe this 3-way interaction with a Taylor Expansion Series equation and generate a 3-dimensional response surface of this interaction. Demonstration of a 3-way interaction between Na-K-Cl may have direct implications in determining the requirements for these nutrients as well as implications to the fields of hypertension and soft-tissue calcification.